and do you really think Singer or Fox would decide to remove all the relationships between these well known x-men instead of changing just a few things here and there?

from a studio point of view, that would be to kill your big franchise.

Do you really think Fox would want to kill the present world of the x-men to just do a sequel set in the 80's and then keep moving just with spin-offs?

which studio would want that?

They're no longer spin-offs.

There are two franchises: Wolverine & the X-Men. Days of Future Past is the second film in this new franchise. First Class was the first. There will be a third and yes it will be set either in the 70s or 80s. The franchise that starred Jackman, McKellen & Stewart is done. The only reason we get to see that expired ensemble once more is that its pivotal to the story Days of Future Past is trying to tell.

The studio isn't killing their new franchise by letting go of the old one. They're using the old one in this film to sell the new one. First Class didn't do well at the box office. Days of Future Past probably will and the third film down the line probably will too.